An electrochemical sensor comprising a supporting material, the supporting material having a noble metal surface, and a homogenous monolayer on the noble metal surface, wherein the monolayer comprises binding molecules which are bound to the noble metal surface via anchor groups, wherein the binding molecules are linked by an ionic bond to at least one charged amino acid residue at the N-terminus, C-terminus, or N- and C- terminus of an enzymatically active protein, and wherein the degree of coverage of the binding molecules on the metal surface is less than the maximum degree of coverage.